1. Field of the Invention
The present invention concerns an apparatus for distribution at a regulated rate of a fluidizable powdery material.
It is known for powdery materials to be transported in the fluidized state from one storage location to another or from a storage location to one or more points of use of the material. It should be noted that the expression "fluidizable materials" as used herein denotes all materials which occur in a powdery form and in respect of which the granulometry and conhesion are such that the speed of flow of the injected air through the material produces, at a low speed, decohesion of the particles from each other and a reduction in the internal frictional forces, in such a way that the suspension so formed behaves very substantially like a homogenous fluid. Such materials are for example alumina which is intended for igneous electrolysis, cements and plasters, quick or slaked lime, fly ash, calcium fluoride, additive fillers for rubber and plastics material, starches, catalysts, powered carbon, sodium sulphate, phosphates, polyphosphates, pyrophosphates, metal powders, plastics materials in the form of powder, foodstuff products such as powdered milk, flour, etc.
2. Related Art
For a very long time now attention has been directed to means for controlling and even regulating the flow rate of a fluidized material (U.S. Pat. No. 2,667,448). Many mechanical constructions (such as valves and control flap arrangements) have been proposed, but they cannot be used when the material being transported is abrasive, which is the case with alumina which is intended for the production of aluminium; in that situation it is necessary to abandon using mechanical arrangements which are subject to wear, and to have recourse to the specific properties of powdery materials.
U.S. Pat. No. 2,919,159 describes a process for the pneumatic transportation of fluidized material, wherein the pneumatic conveyor conduit is fed with powder coming directly from the bottom of a column containing powder which is permanently fluidized with free air.
U.S. Pat. No. 3,964,793 describes a process for regulating the flow rate by weight of powdery materials in an installation for continuous pneumatic transportation by the injection of gas with a balancing effect by means of a fluidized feed column, wherein, for a given gas flow rate by weight, the pressure of the injected gas is measured and the amount of powdery materials introduced into the fluidized column is controlled, the head end of the column being at atmospheric pressure, in order to maintain the injected gas pressure at a given value.
U.S. Pat. No. 4,279,549 describes a process for the automatic regulation of the pneumatic conveyance of powdery material by use of a feed column, at the base of which is formed an embankment portion providing a feed of material by slippage thereof from the embankment portion, a dispatch chamber provided with a porous fluidization wall, conduits for fluidization and feed of gas at an increased pressure, provided with an injector, and conduits intended for transportation of the material. In that process, in order to cause a reduction in the angle of slope of the embankment portion, the material is fluidized and a reference pressure Pf below the porous wall is selected for a desired rate of flow of material into the dispatch duct. Above the porous wall, there is thus established a pressure Pc=Pf in the dispatch chamber, in such a way that any increase in Pc beyond Pf gives rise to a corresponding reduction in the gaseous flow at the pressure Pf through the porous wall while any reduction in Pc below Pf gives rise to a corresponding increase in the gaseous flow.
That process has the advantage of providing for self-regulation of the flow rate of powdery material around the value selected in accordance with the pressure Pf, which eliminates any danger of choking of the dispatch column, and fixes the material flow rate.
At the present time, a certain number of users, in particular operators of Hall-Heroult electrolysis cells for producing aluminium, require such fluidized pneumatic transportation apparatuses to have a service life and a level of reliability which are further increased, together with a higher degree of precision in regard to regulating the material flow rate, both for the purpose of supplying electrolysis tanks and for the purpose of supplying apparatuses for dry-process trapping of fluorine-bearing effluents which are emitted by such tanks, by making use of the capability of adsorption of alumina with respect to fluorine-bearing substances. Finally, it is important that the levels of consumption of compressed air for fluidization and for fluidization and for fluidized conveyance should be minimized and that stoppage and restarting of the appartus occur in a simple, reliable and rapid manner. Such requirements also apply in regard to all types of fluidized conveying operations and not only in relation to alumina which is intended for electrolysis purposes.